84 ON THE IMPURITIES OF COMMERCIAL ZINC. 
tinuing the operation beyond this time (indicated by Otto as sufficient for the testing 
of the materials à blanc), a faint but perceptible mirror gradually formed. ‘This result 
indicates, first, that this sample of Silesian zinc was not perfectly free from arsenic, 
and secondly, that it is dangerous to conclude that the zinc and acid, which have given 
no reaction for arsenic during half an hour in Otto's form of Marsh's apparatus, will 
therefore give no mirror in the next half-hour, even though no arsenical compound be 
added to the apparatus. In any delicate examination for arsenic, this is a point to be 
carefully borne in mind. Our results with this zinc were corroborated by several simi- 
lar experiments. | 
The zinc of Rousseau Fréres was next submitted to the same test, with the same 
acid. 75 grammes of this zinc yielded in half an hour a brown deposit, hardly to be 
called a mirror, although covering a considerable portion (2 centimetres) of the tube. 
The final result of the experiment was not distinguishable from the result of the test 
of Silesian zinc. : | 
. With the same acid and apparatus, 200 grammes of New Jersey zinc gave a distinct 
mirror of arsenic, so large in amount, that the arsenic could be easily recognized by 
its characteristic odor. The mirror began to form at once, and gradually increased 
during the hour, which was the duration of the experiment. 
Having at hand a quantity of the ore from which this zinc is extracted, we extended 
our search for arsenic to the red oxide of zinc, which is the source of this spelter. 
Several grammes of the red oxide, finely powdered, were moistened with 30 drops of 
pure nitric acid, and treated with a measured quantity of pure chlorhydric acid, pre- 
pared from common salt and sulphuric acid free from arsenic. The solution, with the 
very slight- residue, was then gently evaporated to a small bulk, with a small measured 
quantity of Rosengarten’s sulphuric acid. The imperfect solution thus prepared was 
introduced into the flask of our apparatus, whose previous contents of zinc and acid 
had been thoroughly tested for one hour, and found perfectly pure. In ten minutes 
a distinct deposit of arsenic was obtained, which in half an hour increased to a large 
and unmistakable mirror. To prove beyond a doubt that this arsenic came from the 
oxide of zinc, and not from the acids employed in preparing the solution, we tested in 
a clean apparatus with fresh zinc and acid, which had been proved pure by a test of one 
hour in duration, the same quantities of the same acids evaporated together as in the 
experiment above described. At the end of one hour (the second hour during which 
the apparatus had been at work) a deposit, perceptible on close inspection, was dis- 
covered in the narrow part of the reduction-tube. This deposit was, invisible on 
any cursory examination, and bore no comparison with the very decided mirror of ` 
